1. Field of the Invention
This invention relates to a method of flowing a high viscosity substance through a conduit at a low apparent viscosity and more particularly, but not by way of limitation, to a method of flowing a high viscosity hydraulic-fracturing gel in a well fracture at a low apparent viscosity and a high proppant carrying ability.
2. Setting of the Invention
Hydraulic fracturing has made a significant contribution to the petroleum industry as a method for enhancing oil and gas producing rates and recoverable reserves. Fracturing was introduced to the industry in 1949. Since then, it has evolved into a standard operating practice. About 35 to 40 percent of all currently drilled wells are hydraulically fractured, and about 25 to 30 percent of total U.S. oil reserves have been made economically producible by the process. It has increased North America's oil reserves by an additional eight billion barrels.
Over the years, the technology associated with fracturing has increased significantly. A host of fracturing fluids have been developed for reservoirs ranging from shallow, low temperature formations to those that are deep and hot. Many different types of proppants have been developed. These range from silica sand to high strength materials for use in deep formations where fracture closure stresses exceed the ranges of sand capabilities.
A fracturing fluid is used basically to: (1) wedge open and extend a fracture hydraulically, and (2) transport and distribute the proppant along the fracture. The fluid selected for a treatment can have a significant influence on the resulting effectively propped fracture length and fracture conductivity, as well as on 5 treatment costs. Fluid properties strongly govern fracture-propagation behavior and the distribution and placement of the propping agents. The effective viscosity of the fluid controls the internal fracturing pressure and the proppant transporting characteristics.
Cross-linkable fluids are commonly used in fracturing today because of the purportedly better proppant carrying and temperature stability performance. The term "cross-linkable fluids" is used herein to describe fluids which may be crosslinked to form a gel. This cross-linked fluid, or gel, is also referred to as a gel-solid since it may exhibit the infinite viscosity characteristics of a solid, depending upon flowing conditions and shear history. A "delayed" cross-linkable fluid (or delayed crosslinked gel) is one that does not begin crosslinking until reaching a threshold temperature. These cross-linkable fluids have very complex rheological properties and for the most part are guar or hydroxypropyl guar (HPG) solutions crosslinked with some type of metallic compound, e.g., a borate or titanate compound. The cross-linked gel behavior is affected by a number of things, including temperature, temperature history, shear rate and history, time degradation, and chemical contamination. At this time, techniques for characterizing the rheological behavior of cross-linked fluids are not well established. Cross-linked fluids or gels are known to have a very high viscosity and good proppant carrying ability, but their flow curves are not always linear and can demonstrate significant dependence on temperature and shear rate.
Shortcomings of cross-linked fluids include their flow predictability, i.e., the design of fracturing treatments is impeded by the uncertainty in predicting the flow behavior of cross-linked fracturing fluids and it is unknown whether they flow as partially cross-linked solutions, degraded (fractured) gels with similarities to emulsions, or as gels flowing on a fluid layer at the wall. Another shortcoming is the uncertainty in scaling experimental data obtained in lab equipment, such as a Fann viscometer, to field conditions in a well fracture. Another shortcoming is poor experimental repeatability and the large variation in published rheology data which can exist between nominally similar cross-linked fluids provided by different service companies.
Therefore, there is a need for a high viscosity substance with a high proppant carrying ability with predictable and repeatable viscosity and flow characteristics, as well as a method for producing such a substance or fluid. It is contemplated that such an improved fluid and method would produce greater rates of production from wells. It is also contemplated that such a method of flowing a high viscosity substance through a conduit at a low apparent viscosity will provide a means of fracturing a well and carrying proppant at a relatively low pressure, thereby enabling the hydraulic fracturing of wells having low closure stresses which are not fracturable according to conventional practice.